The present invention relates to aircraft instruments, and more particularly, to an FMCW radar system and technique for determining aircraft groundspeed.
One of the more important parameters that is required for aircraft operation and navigation is aircraft groundspeed. During flight, aircraft groundspeed is especially important in determining the flight time between waypoints, in detecting the presence and calculating the speed of head and tail winds, in calculating aircraft position, and in performing other evaluations of aircraft operation. More recently, the measurement of aircraft groundspeed has become inportant in detecting wind sheer so that a pilot may avoid a critical loss of aircraft altitude during flight.
Over the years, a variety of systems and techniques have been developed to provide a measurement of such groundspeed. One exemplary technique involves the use of area navigation equipment including VOR and DME ground stations which provide a signal indicative of aircraft position. Aircraft position can be used to calculate groundspeed by measuring the time change from one position to the next. While this technique provides an approximation of groundspeed, accuracy is limited by the requirement that the aircraft be directly aligned with the particular VOR/DME station. Even when the aircraft is aligned there are still errors which prevent the exact calculation of aircraft groundspeed. Naturally, as the direction of aircraft flight varies from a course which is co-linear with the navigation station, more inaccuracies are introduced into the groundspeed measurement due to the vector relationship of the velocity to flight direction. Accordingly, techniques employing such navigation equipment are at best rough approximations of actual aircraft groundspeed.
In other instances, aircraft groundspeed measurements have been improved by the use of complicated digital systems and techniques which derive groundspeed in a manner related to that described above. Such digital techniques require complicated circuitry for linearizing and filtering high noise signals in order to provide the estimation of aircraft groundspeed. Like the analogous analog systems, the digital techniques rely greatly upon particular flight paths to provide accuracy and are thus only suitable in certain aircraft flight configurations. In addition, the processing needed to produce useful signals requires substantial and sophisticated equipment, adding to the cost and complexity of the system and decreasing its versatility for use in commuter and similar lower cost aircraft.
In one attempt to improve the accuracy of groundspeed measurements, U.S. Pat. No. 4,106,093, assigned to the same assignee as the present invention, utilizes signal inputs from VOR and DME sources and a filter/integrator circuit to produce a groundspeed indicating signal. This technique attempts to provide improved accuracy in the signals representing aircraft position and groundspeed over those previously mentioned attempts using the same VOR/DME stations. Again, the system depends on the presence of VOR/DME signals and, naturally, any inaccessibility to such stations will prevent the acquisition of a groundspeed signal. Accordingly, there is still a need for systems which allow the derivation of groundspeed without reliance upon area navigation stations.
One of the more common techniques employed to determine aircraft groundspeed without the use of VOR/DME and other navigation stations, is that which utilizes a Doppler radar to provide velocity measurement. This technique is widely known and is described in such articles as "The Nature of Doppler Velocity Measurement", IRE TRANSACTIONS ON AERONAUTICAL AND NAVIGATIONAL ELECTRONICS, September 1957, pp. 103-112; "Principles and Performance Analysis of Doppler Navigation Systems", IRE TRANSACTIONS ON AERONAUTICAL AND NAVIGATIONAL ELECTRONICS, December 1957, pp. 176-196; "The Design of Airborne Doppler Velocity Measuring Systems", IRE TRANSACTIONS ON AERONAUTICAL AND NAVIGATIONAL ELECTRONICS, December 1957, pp. 157-175; and "Airborne Doppler Navigation Techniques", Chapter 13 of "Radar Techniques for Detection Tracking in Navigation", PROCEEDINGS OF THE EIGHTH SYMPOSIUM OF THE AGARD AVIONICS PANEL, London, Sept. 21-25 1964. As will be appreciated, the disclosed Doppler systems are capable of providing very accurate measurements of groundspeed without reliance on navigation stations. However, the structure forming such systems is complex and costly which decreases its attractiveness for use in lowercost aircraft. In addition, such Doppler systems typically require at least three non-coplanar radar signals to derive groundspeed thereby further increasing the requirements and complexity of the system. While the measurements may be made with only two beams, a third measurement of vertical velocity derived from some other source (e.g., rate of climb meter) is still required to enable the determination of aircraft groundspeed. Thus, known Doppler techniques are incapable of providing a groundspeed measurement without substantial complex and expensive equipment.
In still another attempt to reduce the complexity of aircraft groundspeed measurement systems, a technique was proposed which allows the velocity to be determined with a radar altimeter echo and non-coherent detection. Radar altimeters, their modes and methods of operation, and their applications, have been well-known for some time as evidenced by such articles including "Radio Altimeter" in the IRE TRANSACTIONS ON AERONAUTICAL AND NAVIGATIONAL ELECTRONICS, June 1954, pp. 3-7; Chapter 8, Sections 1-8 and 10-12 in the book, Frequency Modulated Radar, by G. C. Luck, McGraw Hill, 1949, and "Some Applications of Frequency Modulated Radar" by Irvin Wolf and G. C. Luck, RCA Review, Volume 9, No. 3, 1948. In attempting to provide a velocity measurement using radar altimeter systems, the report entitled "Aircraft Velocity Measurement through Radar Altimeter Echo with Non-Coherent Detection" by C. S. Williams, March, 1974, pp. 1-49, utilizes an altimeter echo received from a leading and trailing antenna located along the longitudinal axis of an aircraft. The antennas, each of which transmits vertically downward from the aircraft and receives an echo from the terrain, are separated by a known distance. When the aircraft is in horizontal flight, the received echo signals can provide an approximation of aircraft groundspeed. This technique assumes the velocity vector of the aircraft is parallel to a line joining the two antennas, and parallel to the ground. In any other flight configuration, the approximation decreases in accuracy. Accordingly, even with the proposed use of less expensive radar altimeter techniques to provide an indication of aircraft groundspeed, there is a continuing need to decrease the complexity of such systems even further and to improve their accuracy for use in a variety of aircraft applications.
Accordingly, the present invention has been developed to overcome the specific shortcomings of the above known and similar techniques and to provide an aircraft groundspeed measurement system and technique using radar altimeters.